Gas compressors are used in a plurality of industrial applications to boost the pressure of a gas, for example for pipeline applications, in the oil and gas industry, in carbon dioxide recovery plants, in compressed air energy storage systems and the like.
The gas processed by the compressor is ingested at an inlet pressure and delivered at a higher outlet pressure, the pressure increase being obtained by conversion of mechanical power into potential pressure energy stored in the gas flow. The process provokes a temperature increase of the processed gas. In some applications the gas temperature can increase to several hundreds of degrees Celsius.
Typical applications where high pressure and high temperature values are achieved by the processed gas are those relating to compressed air energy storage in so-called CAES systems. These systems are used to accumulate energy in form of pressure energy in an air storage cavern, exploiting excess electric power available on the electric distribution grid for example at night time. Typically, multistage gas compressors are used in CAES systems to achieve the required outlet air pressure.
FIG. 1 illustrates a longitudinal section of a multistage centrifugal compressor 100 of the current art. The compressor comprises an outer casing 101, wherein a rotor 103 is housed. The rotor 103 is comprised of a shaft 105 and a plurality of impellers 107. In the example shown in FIG. 1 the multistage centrifugal compressor 100 comprises five impellers sequentially arranged in a flow direction from a compressor inlet 109 to a compressor outlet 111. The shaft 105 is supported by bearings 113, 115.
Each impeller forms part of a respective compressor stage which comprises an inlet channel 117 and a return channel 119. Gas processed by each impeller 107 enters the impeller at the inlet 117 and is returned by the return channel 119 towards the inlet 117 of the next impeller. The return channel of the various compressor stages are formed by one or more diaphragms 121, which are stationarily housed in the casing 101. The gas discharged from the last impeller, i.e. from the most downstream impeller, is collected by a volute 123, wherefrom the compressed gas is conveyed to the gas outlet 111.
The casing 101 can be comprised of a barrel 101B and two end portions 101C, forming a closed housing where the rotor 103 is rotatingly arranged and the diaphragms 121 are stationarily housed.
Mechanical power is used to rotate impellers 107 and is transformed into gas pressure, the pressure increasing gradually as the gas flows through the sequentially arranged impellers. The compression process generates heat so that the gas temperature increases from an inlet temperature to an outlet temperature. The heat is transferred from the gas to the diaphragms 121 and therefrom to the casing 101. The casing 101 therefore is heated up to a maximum steady state temperature, which depends upon the compression ratio of the compressor 100, from the compressor efficiency and from the environment temperature.